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How to join multiple streams and tables with ksqlDB

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How to join multiple streams and tables with ksqlDB

In this tutorial, we demonstrate how to join multiple streams and tables together using an example from retail sales.

Setup

For this example, let's say you have 2 tables customers and items and a stream orders and you want to do a join between all three to enrich the orders stream with more complete information.

Here are the table definitions:

CREATE TABLE customers (customer_id STRING PRIMARY KEY, customer_name STRING)
    WITH (KAFKA_TOPIC='customers',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);
CREATE TABLE items (item_id STRING PRIMARY KEY, item_name STRING)
    WITH (KAFKA_TOPIC='items',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);

And here is the stream definition:

CREATE STREAM orders (order_id STRING KEY, customer_id STRING, item_id STRING, purchase_date STRING)
    WITH (KAFKA_TOPIC='orders',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);

Now, to create an enriched order stream, you'll have an SQL statement like this:

CREATE STREAM orders_enriched AS
  SELECT customers.customer_id AS customer_id, customers.customer_name AS customer_name,
         orders.order_id, orders.purchase_date,
         items.item_id, items.item_name
  FROM orders
  LEFT JOIN customers on orders.customer_id = customers.customer_id
  LEFT JOIN items on orders.item_id = items.item_id;

Running the example

You can run the example backing this tutorial in one of two ways: locally with the ksql CLI against Kafka and ksqlDB running in Docker, or with Confluent Cloud.

Local With Docker

Prerequisites

Run the commands

Clone the confluentinc/tutorials GitHub repository (if you haven't already) and navigate to the tutorials directory:

git clone git@github.com:confluentinc/tutorials.git
cd tutorials

Start ksqlDB and Kafka:

docker compose -f ./docker/docker-compose-ksqldb.yml up -d

Next, open the ksqlDB CLI:

docker exec -it ksqldb-cli ksql http://ksqldb-server:8088

Run the following SQL statements to create the orders stream and customers and items tables backed by Kafka running in Docker and populate them with test data.

CREATE STREAM orders (order_id STRING KEY, customer_id STRING, item_id STRING, purchase_date STRING)
    WITH (KAFKA_TOPIC='orders',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);
CREATE TABLE customers (customer_id STRING PRIMARY KEY, customer_name STRING)
    WITH (KAFKA_TOPIC='customers',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);
CREATE TABLE items (item_id STRING PRIMARY KEY, item_name STRING)
    WITH (KAFKA_TOPIC='items',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);
INSERT INTO items VALUES ('101', 'Television 60-in');
INSERT INTO items VALUES ('102', 'Laptop 15-in');
INSERT INTO items VALUES ('103', 'Speakers');

INSERT INTO customers VALUES ('1', 'Adrian Garcia');
INSERT INTO customers VALUES ('2', 'Robert Miller');
INSERT INTO customers VALUES ('3', 'Brian Smith');

INSERT INTO orders VALUES ('abc123', '1', '101', '2024-09-01');
INSERT INTO orders VALUES ('abc345', '1', '102', '2024-09-01');
INSERT INTO orders VALUES ('abc678', '2', '101', '2024-09-01');
INSERT INTO orders VALUES ('abc987', '3', '101', '2024-09-03');
INSERT INTO orders VALUES ('xyz123', '2', '103', '2024-09-03');
INSERT INTO orders VALUES ('xyz987', '2', '102', '2024-09-05');

Finally, run the stream-table-table join query and land the results in a new order_enriched stream. Note that we first tell ksqlDB to consume from the beginning of the streams.

SET 'auto.offset.reset'='earliest';

CREATE STREAM orders_enriched AS
    SELECT customers.customer_id AS customer_id, customers.customer_name AS customer_name,
           orders.order_id, orders.purchase_date,
           items.item_id, items.item_name
    FROM orders
    LEFT JOIN customers on orders.customer_id = customers.customer_id
    LEFT JOIN items on orders.item_id = items.item_id;

Query the new stream:

SELECT *
FROM orders_enriched
EMIT CHANGES;

The query output should look like this:

+-----------------+-----------------+-----------------+-----------------+-----------------+-----------------+
|ITEMS_ITEM_ID    |CUSTOMER_ID      |CUSTOMER_NAME    |ORDER_ID         |PURCHASE_DATE    |ITEM_NAME        |
+-----------------+-----------------+-----------------+-----------------+-----------------+-----------------+
|101              |1                |Adrian Garcia    |abc123           |2024-09-01       |Television 60-in |
|102              |1                |Adrian Garcia    |abc345           |2024-09-01       |Laptop 15-in     |
|101              |2                |Robert Miller    |abc678           |2024-09-01       |Television 60-in |
|101              |3                |Brian Smith      |abc987           |2024-09-03       |Television 60-in |
|103              |2                |Robert Miller    |xyz123           |2024-09-03       |Speakers         |
|102              |2                |Robert Miller    |xyz987           |2024-09-05       |Laptop 15-in     |
+-----------------+-----------------+-----------------+-----------------+-----------------+-----------------+

When you are finished, exit the ksqlDB CLI by entering CTRL-D and clean up the containers used for this tutorial by running:

docker compose -f ./docker/docker-compose-ksqldb.yml down
Confluent Cloud

Prerequisites

Create Confluent Cloud resources

Login to your Confluent Cloud account:

confluent login --prompt --save

Install a CLI plugin that will streamline the creation of resources in Confluent Cloud:

confluent plugin install confluent-cloud_kickstart

Run the following command to create a Confluent Cloud environment and Kafka cluster. This will create resources in AWS region us-west-2 by default, but you may override these choices by passing the --cloud argument with a value of aws, gcp, or azure, and the --region argument that is one of the cloud provider's supported regions, which you can list by running confluent kafka region list --cloud <CLOUD PROVIDER>

confluent cloud-kickstart --name ksqldb-tutorial \
  --environment-name ksqldb-tutorial \
  --output-format stdout

Now, create a ksqlDB cluster by first getting your user ID of the form u-123456 when you run this command:

confluent iam user list

And then create a ksqlDB cluster called ksqldb-tutorial with access linked to your user account:

confluent ksql cluster create ksqldb-tutorial \
  --credential-identity <USER ID>

Run the commands

Login to the Confluent Cloud Console. Select Environments in the lefthand navigation, and then click the ksqldb-tutorial environment tile. Click the ksqldb-tutorial Kafka cluster tile, and then select ksqlDB in the lefthand navigation.

The cluster may take a few minutes to be provisioned. Once its status is Up, click the cluster name and scroll down to the editor.

In the query properties section at the bottom, change the value for auto.offset.reset to Earliest so that ksqlDB will consume from the beginning of the streams we create.

Enter the following statements in the editor and click Run query. This creates the orders stream and customers and items tables and populates them with test data.

CREATE STREAM orders (order_id STRING KEY, customer_id STRING, item_id STRING, purchase_date STRING)
    WITH (KAFKA_TOPIC='orders',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);

CREATE TABLE customers (customer_id STRING PRIMARY KEY, customer_name STRING)
    WITH (KAFKA_TOPIC='customers',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);

CREATE TABLE items (item_id STRING PRIMARY KEY, item_name STRING)
    WITH (KAFKA_TOPIC='items',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);

INSERT INTO items VALUES ('101', 'Television 60-in');
INSERT INTO items VALUES ('102', 'Laptop 15-in');
INSERT INTO items VALUES ('103', 'Speakers');

INSERT INTO customers VALUES ('1', 'Adrian Garcia');
INSERT INTO customers VALUES ('2', 'Robert Miller');
INSERT INTO customers VALUES ('3', 'Brian Smith');

INSERT INTO orders VALUES ('abc123', '1', '101', '2024-09-01');
INSERT INTO orders VALUES ('abc345', '1', '102', '2024-09-01');
INSERT INTO orders VALUES ('abc678', '2', '101', '2024-09-01');
INSERT INTO orders VALUES ('abc987', '3', '101', '2024-09-03');
INSERT INTO orders VALUES ('xyz123', '2', '103', '2024-09-03');
INSERT INTO orders VALUES ('xyz987', '2', '102', '2024-09-05');

Now, paste the stream-table-table join query in the editor and click Run query. This will land the results in a new orders_enriched stream.

CREATE STREAM orders_enriched AS
    SELECT customers.customer_id AS customer_id, customers.customer_name AS customer_name,
           orders.order_id, orders.purchase_date,
           items.item_id, items.item_name
    FROM orders
    LEFT JOIN customers on orders.customer_id = customers.customer_id
    LEFT JOIN items on orders.item_id = items.item_id;

Query the new stream:

SELECT *
FROM orders_enriched
EMIT CHANGES;

The query output should look like this:

+-----------------+-----------------+-----------------+-----------------+-----------------+-----------------+
|ITEMS_ITEM_ID    |CUSTOMER_ID      |CUSTOMER_NAME    |ORDER_ID         |PURCHASE_DATE    |ITEM_NAME        |
+-----------------+-----------------+-----------------+-----------------+-----------------+-----------------+
|101              |1                |Adrian Garcia    |abc123           |2024-09-01       |Television 60-in |
|102              |1                |Adrian Garcia    |abc345           |2024-09-01       |Laptop 15-in     |
|101              |2                |Robert Miller    |abc678           |2024-09-01       |Television 60-in |
|101              |3                |Brian Smith      |abc987           |2024-09-03       |Television 60-in |
|103              |2                |Robert Miller    |xyz123           |2024-09-03       |Speakers         |
|102              |2                |Robert Miller    |xyz987           |2024-09-05       |Laptop 15-in     |
+-----------------+-----------------+-----------------+-----------------+-----------------+-----------------+

Clean up

When you are finished, delete the ksqldb-tutorial environment by first getting the environment ID of the form env-123456 corresponding to it:

confluent environment list

Delete the environment, including all resources created for this tutorial:

confluent environment delete <ENVIRONMENT ID>
Do you have questions or comments? Join us in the #confluent-developer community Slack channel to engage in discussions with the creators of this content.